1. Field of the Invention
The present invention relates to an image-forming apparatus using an electrophotographic system or an electrostatic recording process, and in particular relates to an image-forming apparatus such as a copying machine, a printer, and a facsimile machine, or to a multi-function machine having a plurality of these functions.
2. Description of the Related Art
An image-forming apparatus, such as a copying machine, a printer, and a facsimile machine, have widely used a method in which an unfixed toner image is formed on a recording material (a paper member such as transfer paper, photographic paper, dielectric-coated paper, and printing paper) formed thereon by a transfer system (indirect system) or a direct system with an appropriate image-forming mechanism, such as an electrophotographic system, an electrostatic recording system, and a magnetic recording system; then, the unfixed toner image is thermally fixed.
Recently, in such an image-forming apparatus, various kinds of recording material has been required in addition to so-called copy paper. As an example, there is cardboard: in order to fix an image on cardboard you require large thermal capacity, and a pressurizing force applied to a fixing roller is an important condition as well as thermal capacity applied to the recording material, so that the pressurizing force also needs to be increased.
However, as a negative effect of the increased pressure, the drive load violently increases at the moment when the leading edge of the cardboard is introduced into the nip between the fixing rollers. As a result, rotational variations of the fixing rollers are liable to be produced because of the rotational variation of a motor itself and the accumulation of micro-deflections of components of a drive system produced if the drive transfer path to the fixing rollers. FIG. 18 is a graph showing rotational variations of a fixing roller 4a plotting time t in abscissa and peripheral speed V of the fixing roller 4a in ordinate, and symbol A denotes the rotational variations when the cardboard is introduced into the nip of the fixing roller 4a. As shown in the drawing, the peripheral speed is reduced by the violently increased load. When the fixing roller 4a rides out the leading edge, the load is decreased; however, the peripheral speed is increased higher than a steady speed by the reaction of the drive system, which is deflected when the load is increased.
Then, the rotational variations are attenuated so that the steady speed is returned: at this time, the following problems arise.
Japanese Patent Laid-Open No. 6-318009 discloses that in order to prevent the speed of a fuser from varying when paper runs into the fuser, the drive system of the fuser may be provided with a flywheel.
Japanese Patent Laid-Open No. 2000-19798 discloses a magnetic clutch is provided for selectively connecting between a drive system of a fuser and a flywheel, so that when a recording material is jammed, the connection between the drive system of the fuser and the flywheel is released so as to reduce a period of time required for stopping the fuser.
A full-color image-forming apparatus is provided with a device for applying silicone oil as release agent on a fixing roller in order to improve releasability of a transfer agent from the fixing means and to prevent jamming.
In such a device, since it is driven by the same drive source as that of the fixing roller 4a, when the fixing roller varies in rotation, an oil applying roller 40 also varies in rotation. At this time, oil coating unevenness may be produced, resulting in uneven brightness and streaks of images. An example of such a device will be described below.
Referring to FIG. 15, the fixing roller 4a and the pressure roller 4b, which are for fusing the recording material in the nip by pressurizing and conveying it, are made of an aluminum core having elastic silicone rubber fixed on the surface; inside each core, halogen heaters 45a and 45b are arranged as heating sources; and thermistors (not shown) come in contact with on the respective surfaces of the silicone rubber for detecting temperatures of the surfaces. The controller in the body controls the electric power supply to the halogen heaters by comparing the temperature of each thermistors with a pre-established temperature so as to turn on the halogen heaters 45a and 45b via an AC driver whenever the temperature is lower than the established one while turning off electric power supply whenever the temperature is higher than the established one, thereby controlling the temperatures of the fixing roller 4a and the pressure roller 4b to maintain them constant.
In the device shown in the drawing, part of a first pumping roller 43 is dipped in release oil O contained in an oil pan 65 while a second pumping roller 42 rotates adjoining the first pumping roller 43. A third pumping roller 41 is adjacent to the second pumping roller 42. The second pumping roller 42 is rotated by a drive source so as to transmit the driving force to a coating roller 40 for applying the release oil O via the third pumping roller 41 and the first pumping roller 43. The second pumping roller 42 also serves as the rotational center for urging the coating roller 40 to the fixing roller 4a by a pressure spring 38. In such a manner, the coating roller 40 applies the release oil O sequentially pumped by each of the pumping rollers on the surface of the fixing roller 4a. A metering blade 44 for restricting the release agent made of an elastic body, such as fluoride rubber, and urged by a spring 49 so as to constantly maintain a predetermined pressure is arranged in contact with the coating roller 40, thereby defining the amount of the oil on the coating roller 40 at a predetermined value. The oil on the coating roller 40 restrictedly applied thereon is transferred to the fixing roller 4a. The residual oil on the coating roller 40 is removed outside the oil pan 65 by a cleaning blade 39 together with toner and paper dust stuck on the fixing roller 4a. This residual oil is recovered to an oil tank (not shown), which will be described later, via a filter for recycling.
Details will be described with reference to FIGS. 15 to 17. As described above, the oil transferred to the pumping roller 41 from the pumping roller 43 and the oil pan 65 is restricted by the metering blade 44 on the coating roller 40. The restricted oil forms a minute oil sump D between the edge of the metering blade 44 and the coating roller 40 along the entire longitudinal region during the steady rotation. The oil sump D is formed to keep the balance between the amount of oil scraping through the metering blade 44, a surface tension, an oil viscosity, and the gravity. However, if vibration, such as rotational variations mentioned above, is applied, the oil sump D gets out of balance so that the oil pours out. The poured out oil is transferred on the recording material in a streak pattern via the fixing roller 4a, and the length of the streak from the leading edge of the recording material is equivalent to distance L between the metering blade 44 and the nip of the fixing roller 4a (FIG. 16). The oil streak is not noticeable in images if the amount is small; however, if the amount is increased, the oil streak affects images as uneven brightness and color shading, and this effect deteriorates in proportion to the intensity of the vibration, such as the rotational variations.
However, when the flywheel is provided in the driving system of the fuser in order to suppress the rotational variations due to the recording material running into the nip of the fuser, since the flywheel must be rotated even for fixing images on thin paper having small rotational variation when running into the fuser, vibration due to the decentration of the flywheel and dispersion of the weight may be generated, deteriorating banding.
In a small sized device with a small distance between the transfer and the fixing, the vibration also may especially affect the vicinity of an engine through a frame. Although the vibration is reduced if the flywheel and the drive transfer system are manufactured with high precision, this device has a defect of high cost including assembling requiring adjustment. When the fuser is started in a state that the flywheel is connected thereto, since the inertial load due to the flywheel is large, a motor larger than necessary must be used especially in using a stepping motor as a drive source.